The appearance of a colored object is affected by three primary factors: the photometric characteristics of the light source which illuminates the color, the photometric characteristics of the colored object, and the perception of these characteristics by an observer. Additional factors such as texture, gloss, etc. may affect how a color is perceived by an observer, but have varying affects on the spectrophotometric characteristics of that color.
Colorimetric systems have been developed in an effort to objectively describe how colors are perceived by observers. The International Commission on Illumination (CIE) has, for example, developed a system for objective description of color by light source, object, and observer. The CIE developed a standardization of the illuminant and observer data. The color of the object under these standard conditions is identified by tristimulus values X,Y and Z. Each XYZ value is obtained by multiplying the reflectance of the colored sample, the power of the standard illuminant, and the calculated amount of each of the three primary colors (red, green, and blue) which, when combined, are found through observational tests to be the color equivalent of the object being described.
The tristimulus values X,Y, and Z are of somewhat limited value as color specifications because
they do not correlate well to visual attributes. As a result, the CIE adopted the use of chromaticity coordinates x, y, and z which are the amounts of each tristimulus value divided by the sum of all three.
The CIE chromaticity calculations have been further transformed by the use of the L.sup.* a.sup.* b transformation. The L.sup.* a.sup.* b system identifies color by lightness or darkness as its L.sup.* value. The saturation, or amount of dullness or brightness (deviation from gray) a color has, as well as hue, or what is commonly called color (blue, green, etc.), are both identified using +a, +b values. The L.sup.*, a.sup.*, and b values may be used as coordinates to lay out the color system in a three-dimensional space. Typically, the L.sup.* value is shown in the vertical z-direction. The +a.sup.*, +b values are set in the XY plane. Plus a is red, -a.sup.* is green, +b is yellow, and -b is blue. L.sup.* =0 is black, and L.sup.* =100 is white. Between these extremes of each value, all colors can be identified. See FIG. 1.
In addition to the above-described color description systems, a number of other color order systems are commercially used to identify color samples and for other purposes. Among the most popular is the Munsell system. The Munsell system has a lightness value essentially corresponding to the L.sup.* value of the L.sup.* a.sup.* b system. Unlike the L.sup.* a.sup.* b system, hue and saturation are defined by a letter-number system corresponding to the hue and chroma characteristics of the color.
It would be desirable for a commercial color sample system to have several characteristics. Among them are that the system have representations and be layed out in a geometry which is generally consistent with the typical user's intuitions as to the components and relationships of the represented colors. Moreover, a system adapted to the L.sup.* a.sup.* b system would have advantages including ease of transformation between systems and ease of access in processing data available through the L.sup.* a.sup.* b system.
Colors subject to metamerism may have other types of characteristics which are identical. A system which only identifies colors as similar when there is no metamerism between the colors is desirable. A color sample system would be desirable that simplifies matching efforts between a sample color and a color to be manufactured from a given set of colorants identifying the color sample in a manner readily processible by a system which determines how to mix the available colorants. Ideally, such matching could be accurately achieved even for sample colors not identified in the system through an objective description of the color based on identified sample colors.